Heretofore it has been the practice to embed steel reinforcing in concrete and establish an intimate bond between the reinforcing and concrete throughout the entire surface of the reinforcing. This practice has been followed in the techniques of pretensioning and posttensioning of concrete. In the pretensioning technique, the reinforcing tendons are held in tension while the concrete is poured around the reinforcing. In the posttensioning technique, the reinforcing tendons are jacketed in sleeves large enough to permit sliding movement of the tendons. Then, after the concrete is set, the tendons are tensioned, grouting is forced between the tendons and the sleeves so that the grouting is bonded to the tendons and the tendons are anchored at their extremities.
In all cases, there is a continuous intimate bond between the reinforcing or tendon and the concrete or grouting. If the concrete or grouting cracks, localized stress is applied to the reinforcing or tendon; that is, the reinforcing or tendon is stressed over the width of the crack (plus some relatively small bond length at each side of the crack). As a result, the crack need only widen a small amount to stretch the short length of the reinforcing or tendon beyond the yield point of the steel. If the force which caused the crack is relieved, the reinforcing or tendon does not recover, and the reinforcing or tendon exerts at least a local force tending to widen the crack.
Should the force which creates the crack or a later force be of sufficient magnitude, the crack may widen sufficiently that the localized stress in the reinforcing or tendon exceeds the ultimate strength of the steel and a rupture occurs.
This problem is fully recognized so that in designing a concrete structure using ordinary reinforcing or tendon reinforcing, care is taken that the expected loads will be insufficient to produce the cracks. This, of course, materially increases the cost of construction, especially if the designer must consider gross overloads such as produced, for example, by earthquakes or falling aircraft.